1. Field of the Invention
The present invention relates to a communication system, and in particular, to a communication system which uses a differential two-wire type of communication line serving as a communication line composing a network.
2. Description of the Related Art
Recent vehicles are provided with in-vehicle communication systems with which a variety of in-vehicle devices are controlled. In such in-house communication systems, a communication system using a differential two-wire type of communication line is familiar, which composes a network in the vehicle.
This type of communication system adopts a network provided with a communication line generally consisting of a main line serving as a main transmission path and a plurality of branch lines branched from the main line. A node is connected to each branch line. Both ends of the main line are terminated by termination circuits.
In cases where branch lines are equipped in the communication system, a mismatch between characteristic impedances on the communication lines will cause reflection of electromagnetic waves. Such a mismatch exists at a branch point from which each branch line is branched from the main line (that is, each branch point is a connection point of both each branch line and the main line). The reflected waves will distort signal waves of frames of data to be transmitted. Depending on how much the distortion occurs, signal reception may result in failure. That is, an addressed node may receive erroneous data.
In consideration of such circumstances, Japanese Patent Laid-open Publication No. 9-238164 discloses a technique for suppressing reflected waves from occurring in each branch line. Practically, a resistor and a capacitance, both for matching impedances, are interposed between the main transmission path and each branch wiring, in which the resistor and capacitance are made to partly be valid during transmission of frame data and made to be invalid after the transmission of the frame data.
However, although the suppression technique disclosed by the above reference is helpful for suppressing the occurrence itself of reflected waves, it has no capability of dismissing the once-occurring reflected waves. in other words, by this suppression technique, electromagnetic waves through the transmission lines can be suppressed from being reflected during the transmission of frame data, but it is not effective for reducing the reflected waves occurring immediately after completion of the sending-out operation for frame data.
In the case of the above-described conventional technique, it is difficult to sufficiently avoid the erroneous data reception. In contrast, it is required to limit the degree of freedom for designing the lengths of communication lines and the topology, if the impedance matching is made to focus a state where the communication lines are not subjected to the transmission of frame data. These mutual-conflicting difficulties are especially concerned about in large-size networks, because there are a large number of branches in such networks and reflected waves occurring on the communication lines are more severe.